1. Field of the Invention
The present invention relates to an expansion tube for manufacturing a heat shrinkable tube, and more particularly, to an expansion tube with a plurality of expansion slits formed thereon and an apparatus for manufacturing a heat shrinkable tube having the expansion tube, whereby the linear velocity of expansion of the heat shrinkable tube can be substantially increased, the cooling efficiency of the heat shrinkable tube can be enhanced, the friction of the heat shrinkable tube against the expansion tube can be substantially reduced, and the deviation of elongation of the heat shrinkable tube after expansion can be greatly lowered.
2. Background of the Related Art
Generally, a heat shrinkable tube is a product that is made of a polymeric synthetic resin that shrinks to a predetermined percentage if it is heated. The heat shrinkable tube is made of a thermoplastic polymeric synthetic resin that is composed of molecules that are long and thin, arranged in irregular fashions. Especially, crystals are formed between the two adjacent molecules to each other, which provides a force of coupling between them to the molecules.
The thermoplastic material is changed in shape if heated, but it is still kept in the changed shape once if cooled. That is to say, if the thermoplastic material is cooled after heating, it has got the crystals regenerated. Thus, when the crystals regenerated are processed by the irradiation with a high energy of radiation, permanent crosslinking is provided between the adjacent molecules. The crosslinked material is generally flexible and even if it is heated at a temperature over a melting point of the crystals, it is not melt and changed only in shape.
Specifically, the crosslinked tube has an elastic memory property with which it can remember its original shape before the change. In accordance with the types of product, thus, the heat shrinkable tube having the elastic memory shrinks in a radial direction to a range of 25% to 75% and shrinks in a longitudinal direction to a range of 10% or less.
Such the heat shrinkable tube is adapted to the product to be manufactured, for the purpose of being used in the applications where the electrical insulation or corrosion protection is required.
FIG. 1 is a schematic view of a conventional apparatus for manufacturing a heat shrinkable tube, wherein the apparatus includes a denser 2 that is adapted to receive a heat shrinkable tube 1, while preventing the heat shrinkable tube 1 from being loose, dry heating rolls 3 that are adapted to heat the heat shrinkable tube 1 ejected from the denser 2, an expansion chamber 4 that is adapted to expand the heat shrinkable tube 1 ejected from the dry heating rolls 3, a guide roller 5 that is adapted to guide the heat shrinkable tube 1 ejected from the expansion chamber 4 to a predetermined direction, a caterpillar 6 that is adapted to have the heat shrinkable tube 1 constant in diameter, and a bobbin 7 that is adapted to wind the heat shrinkable tube 1 ejected from the caterpillar 6.
The denser 2 serves to prevent the heat shrinkable tube 1 from being loose by the use of the dry heating rolls 3 before the expansion of the tube and also keeps the tube 1 from being cut.
The dry heating rolls 3 are disposed to heat the heat shrinkable tube 1 after passing the denser 2 to a predetermined temperature, such that the heat shrinkable tube 1 can be placed to be changeable in shape. Next, the heat shrinkable tube 1 is moved to the expansion chamber 4.
FIG. 2 is a sectional view of the conventional expansion chamber 4 employed for manufacturing the heat shrinkable tube 1. The expansion chamber 4 includes a given size of hollow body 4-1, an expansion tube 10 having a plurality of substantially circular expansion holes 11 regularly arranged in a lengthwise direction on the outer peripheral surface thereof, being contacted with the heat shrinkable tube 1 coming through a Teflon adapter 12, a plurality of cooling water nozzles 22 disposed on the surface of the body 4-1, for spraying cooling water through the plurality of expansion holes 11 such that the heat shrinkable tube 1 becomes cooled, a cooling water pump 24 for supplying the cooling water to the plurality of cooling water nozzles 22, a vacuum suction hole 21 disposed on the surface of the body 4-1, for discharging the air in the expansion tube 10 through the plurality of expansion holes 11 and the air in the body 4-1 to the outside of the expansion chamber 4, and a vacuum pump 23 connected to the vacuum suction hole 21, for discharging the air in the expansion chamber 4 to the outside.
FIG. 3 is a front view of the conventional expansion tube 10. The expansion tube 10 is provided with the plurality of substantially circular expansion holes 11 regularly arranged on the outer peripheral surface thereof along a longitudinal direction. The heat shrinkable tube 1 that is passed through the Teflon adapter 12 is placed under a uniform environment in the expansion tube 10.
An explanation of the steps of processing the heat shrinkable tube 1 through the expansion tube 10 under the aforementioned construction is given in more detail.
First, the heat shrinkable tube 1 is heated to a temperature range from 120° C. to 250° C. through the dry heating rolls 3 and then, it is moved into the expansion tube 10 through the Teflon adapter 12. In this case, there is a space in which a small amount of air flows out between the Teflon adapter 12 and the heat shrinkable tube 1 such that the heat shrinkable tube 1 is moved into the expansion tube 10, not being directly contacted with the Teflon adapter 12.
When the heat shrinkable tube 1 is moved into the expansion tube 10, the air in the expansion chamber 4 is discharged to the outside through the vacuum suction hole 21 connected with the vacuum pump 23, which causes the pressure in the expansion chamber 4 to be lowered. When the air in the expansion chamber 4 is sucked through the vacuum pump 23, the air in the expansion tube 10 is evenly discharged through the plurality of expansion holes 11 such that the heat shrinkable tube 1 in the expansion tube 10 is expanded to larger radius than that before moved into the expansion tube 10.
At that time, when the cooling water nozzles 22 disposed on the surface of the body 4-1 are operated to cool the heat shrinkable tube 1 whose radius is enlarged, the cooling water is supplied from the cooling water pump 24 and sprayed into the expansion chamber 4. The sprayed cooling water comes into the expansion tube 10 via the plurality of expansion holes 11 and is applied to the outer peripheral surface of the expanded heat shrinkable tube 1. Thus, the heat shrinkable tube 1 becomes cooled in the state of being enlarged in radius.
The heat shrinkable tube 1 after cooling is moved out of the expansion chamber 4 and is wound on the bobbin 7 through the guide roller 5 and the caterpillar 6.
However, each of the expansion holes 11 of the conventional expansion tube 10 is circular in shape and small in size, as shown in FIG. 3, so that the linear velocity of expansion of the heat shrinkable tube 1 is relatively slow, which makes the time of manufacturing the product substantially extended. On the other hand, in case of making the linear velocity of expansion of the heat shrinkable tube high, the cooling water fails to completely cool the expanded heat shrinkable tube. Therefore, the heat shrinkable tube 1 takes an irregular shape.
To solve the problems, there is provided an expansion tube that has relatively large expansion holes, but when the air in the expansion camber is discharged by means of the vacuum pump, the portion of the heat shrinkable tube that is placed in each of the large expansion holes is expanded outwardly more than the inner peripheral surface of the expansion tube over the expansion hole such that the expanded heat shrinkable tube becomes irregular on surface. Therefore, the expansion tube exhibits a high cooling effect to the heat shrinkable tube but when the heat shrinkable tube is finally made, it gets the irregular heat shrinkable tube.